Various exhaust emission control technologies are used for treating exhaust gases. One such technology is selective catalytic reduction (SCR). In a typical SCR system a reducing agent is supplied upstream of a catalyst positioned in an exhaust passage, through an injector. The reducing agent reacts with NOx present in the exhaust gases in the presence of catalyst to convert NOx to N2 and water. It is important for the reducing agent to be injected at regular intervals for efficient working of the SCR.
Some SCR systems include a pump for supplying the reductant from the reductant storage to the injector, while others utilize pressurized air. SCR systems using pressurized air need a constant supply of pressurized air from an air source. One such air source may be a turbocharger that provides pressurized air to facilitate injection of the reducing agent and is driven by the exhaust gases. In certain instances, like extended engine-idling periods or periods with low engine loading, the amount of exhaust gases is reduced than the amount of exhaust gases produced during periods with normal or high engine loading. Consequently, the turbine of the turbocharger rotates slower and the pressure of the pressurized air discharged by the compressor is reduced. This results in situations where the pressure of the pressurized air does not meet the minimum pressure required for the injection of the reducing agent. Thus, the SCR system is rendered ineffective due to irregular, or no injection of DEF.
U.S. Pat. No. 7,765,793 discloses an exhaust emission control device for controlling a nozzle opening degree of the variable geometry turbocharger based on a result detected by a load detector. If the load detected is in a low load range, the nozzle opening degree is controlled such that the supercharging pressure is at or above a predetermined value at which the reducing agent is atomized. The delivery of reductant is controlled by a dedicated pump, which adds complexity and potential failure points to the system.